Continuous Annealing Lines (hereinafter, referred to as “CALs”) are typically used in the heat treatment of steel strip. Vertical CALs and Continuous Galvanizing Lines (hereinafter, referred to as “CGLs”) are ordinarily divided into respective sections of a heating furnace, a soaking furnace and a cooling furnace. A predetermined heat treatment cycle is performed by controlling the furnace temperature and the time the strip spends in each furnace section, which is commonly referred to as the strip speed. The heat treating sections are equipped with furnace rolls to allow a continuous annealing and/or a continuous galvanizing process. The furnace rolls act as transfer rolls located on the upper portion and the lower portion of the vertical heat treating sections, and a metal strip is passed while being suspended by these furnace rolls and subjected to a necessary time at temperature in a specific atmosphere. These rolls typically operate in temperatures ranging from 600° C.-1200° C. in a reducing atmosphere converting iron oxide to metallic iron. The rolls must be able to maintain the capability to convey the steel strip at high temperatures over long periods of continuous operation. However, as a result of the severe operating conditions, including, for example, faster line speeds, higher temperatures and the treatment of Advanced High Strength Steel (hereinafter, referred to as “AHSS”), the rolls can be subject to several potential problems, including wear of the roll surface and loss of the roll profile. Furthermore, adhesion of oxide or iron dust type particulate matter to the roll surface may be transferred from the strip to the roll during operation. The accumulation of adhesive material onto a roll surface is typically referred to as “pick-up”. Pick-up of adhered matter can accumulate to a level where the furnace rolls are unable to convey and transfer the steel strip without creating quality blemishes on the strip surface. Accordingly, the furnace must be shut down so that the surface of the rolls can be cleaned or replaced. A furnace shut down causes a significant loss in terms of steel production. Additionally, vast quantities of energy are lost when the furnace is opened and allowed to cool for repair and then reheated to the annealing temperature. It is normal for a line to shut down once or twice a year for scheduled maintenance, but unscheduled shut-downs contribute to higher costs for steel producers.
As a means to delay the onset of the pick-up phenomena, it is well known to apply a protective coating layer on the surface of the rolls. For example, a combination ceramic with a metal alloy—can be used. Carbide materials can be used at low temperatures. Ni-based or Co-based alloys containing Cr and other alloying elements are used as the metal matrix so the mismatch in the coefficient of thermal expansion (CTE) between the heat resistant steel roll base and a pure ceramic does not lead to a short coating life (e.g., coating life limited by coating cracks and delamination due to the stresses form by CTE mismatch).
MCrAlY materials (where M represents metal that can include cobalt (Co), nickel (Ni) and/or iron (Fe); Cr represents chromium; Al represents aluminum; and Y represents yttrium) blended or loaded with ceramic materials have also been applied to high temperature furnace rolls to resist Fe and FeO transfer. Such materials have been used since the late 1970's in heat treatment furnaces, including, for example, a horizontal CAL. These coatings have also been incorporated into vertical CALs and CGLs since the early 1980's. Vertical CAL and CGLs today have larger furnace rolls and run at faster line speeds requiring the protective MCrAlY coating to not only resist the transfer or pick-up of Fe/FeO, but also to resist wear of the roll surface from the steel strip. Furnace rolls that are coated with MCrAlY materials containing approximately 10 weight percent ceramic loaded therein have performed well (exhibiting a typical life of 5-10 years) because the strip was made from mild or high strength low alloy (hereinafter, referred to as “HSLA”). Examples of such coated rolls are included in U.S. Pat. No. 4,124,737, which discloses MCrAlY loaded with 10 wt. % ceramic, and U.S. Pat. No. 4,822,689.
However, in the last ten years, the emergence of advanced high strength steel (hereinafter, referred to as “AHSS”) presents new design challenges for protective MCrAlY coatings. The AHSS contains significantly more alloying elements (e.g., Al, Si, Mn and Ti) in comparison to mild strength steel or HSLA. The increased concentrations of alloying elements in AHSS have necessitated an influx of new generation MCrAlY coatings. Prior art efforts to solve this problem have been attempted as disclosed in U.S. Pat. No. 6,572,518, but further improvements are needed as the challenges associated with AHSS significantly increase.
Notwithstanding the newly designed MCrAlY formulations and coatings, the problem of pick-up remains prevalent. The adhered matter continues to accumulate to a level where the furnace roll pick-up causes quality blemishes on the steel strip, thereby causing the furnace to shut down for cleaning and smoothening of the surface of the rolls. Furthermore, steel strip used for automotive stampings has become thinner for weight savings, thereby allowing better automotive fuel economy. At the same time, the quality requirements have increased, whereby automotive customers are requiring premium finishes on the exposed body panels. Therefore, even reduced levels of pickup will impart blemishes or dents on the thinner strip, leading to quality issues and ultimately rejected strip.
In view of the problems associated with adhered matter pick-up on furnace rolls in the steel industry, there is an unmet need for an improved composite coating that would render oxidative and corrosive protection while resisting spallation, and also reduce and delay the incidence of buildup on furnace roll surfaces, thereby allowing higher quality steel strip and improved automotive finishes on exposed body panels.